test: fix flaky TestResourceIndex and reduce its runtime (#37847)

The modernc SQLite driver (default since
https://github.com/go-gitea/gitea/pull/37562) returns `SQLITE_BUSY` once
the busy timeout is reached, unlike mattn which waited indefinitely.
`TestResourceIndex` fires many concurrent `NewIssue` writers, but SQLite
serializes all writers, so they queue on a single `BEGIN IMMEDIATE`
write lock. Under `-race` (modernc is much slower) the goroutines at the
back of the queue exceeded the hardcoded 5s test timeout, producing
`database is locked (5) (SQLITE_BUSY)`.

Changes:
- Reduce the concurrent inserts from 25 to 10. Since SQLite serializes
writers, the extra goroutines only deepen the busy-lock queue without
adding coverage. 10 still exercises concurrent index allocation while
cutting the test's `-race` runtime ~3x (2.76s to 0.86s locally).
- Share the busy-timeout constant: export `DefaultSQLiteBusyTimeout`
(20s, the production default) and reference it from the test engine
instead of the hardcoded `5000`.

Observed flake:
https://github.com/go-gitea/gitea/actions/runs/26394082930/job/77690496092

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

models/issues/issue_test.go

@@ -299,7 +299,7 @@ func TestIssue_ResolveMentions(t *testing.T) {
 func TestResourceIndex(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 	var wg sync.WaitGroup
-	for i := range 25 {
+	for i := range 10 {
 		wg.Go(func() {
 			testInsertIssue(t, fmt.Sprintf("issue %d", i+1), "my issue", 0)
 		})

models/unittest/testdb.go

@@ -207,7 +207,7 @@ type FixturesOptions struct {
 
 // CreateTestEngine creates a test database and loads the fixture data from fixturesDir
 func CreateTestEngine(testSQLiteFile string, opts FixturesOptions) error {
-	driver, connStr, err := db.ConnStr(db.ConnOptions{Type: setting.DatabaseTypeSQLite3, SQLitePath: testSQLiteFile, SQLiteBusyTimeout: 5000})
+	driver, connStr, err := db.ConnStr(db.ConnOptions{Type: setting.DatabaseTypeSQLite3, SQLitePath: testSQLiteFile, SQLiteBusyTimeout: setting.DefaultSQLiteBusyTimeout})
 	if err != nil {
 		return err
 	}